Spas, hot tubs, pools, hydrotherapy pools, bath tubs, and similar bodies of water used indoors, outdoors, or both indoors and outdoors are used for both therapeutic and recreational purposes (all forms of the aforementioned and derivatives thereof are referred to hereinafter as “spas”). When used for these purposes, the spa water is typically heated from ambient temperature to a desired temperature of approximately 90 to 120 degrees Fahrenheit. Because spas contain a large amount of water that must be heated rather rapidly, various types of water heaters have been used. Due to extensive building safety code regulations and high initial setup costs for gas heating water for spas, the majority of spas use heaters that employ electric heat in some form or fashion.
Recent trends in the industry have been to use one of three general methods to electrically heat spa water. The first method is to have an electrical heating element in the piping system or in an enlarged portion of the piping system to heat the water as it flows through the pipe and comes into contact with the heating element. Examples of this heating method are disclosed in U.S. Pat. No. 5,978,550, issued Nov. 2, 1999, invented by Rochelle, entitled WATER HEATING ELEMENT WITH ENCAPSULATED BULKHEAD; U.S. Pat. No. 5,438,712, issued Aug. 8, 1995, invented by Hubenthal, entitled HOT TUB HEATER SYSTEM; and U.S. Pat. No. 6,080,973, issued Jun. 27, 2000, invented by Thweatt, entitled ELECTRIC WATER HEATER. These are very efficient methods of heating spa water due to the heating element being surrounded by spa water, which dissipates the majority of heat produced into the spa water. However, the reason for this method's efficiency is also the reason for its frequent failure and need for repairs. Because the heating element is surrounded by chemically treated water at high temperatures, the heating element is subject to various types of corrosion, including: galvanic corrosion, chemical pitting, intergranular corrosion, stress corrosion cracking, corrosion fatigue, electrochemical corrosion, and bacterial corrosion due to Ferrobacillus bacteria. This corrosion exposure is one of the most common and most frequent causes of spa breakdown, which generally requires a costly repair due to pipes needing to be cut to expose the heating element, or replacement of the entire heater apparatus. Furthermore, this method is prone to leaks and failures due to the need for bulkheads to allow the electric line(s) to pass from the outer-dry surface to the inner-wet surface, so the heating element can be surrounded by the water that is to be heated. The bulkheads are another common source of failure in spa heaters, which make them susceptible to leaks and water intrusion.
The second method of heating spa water is to have an electrical heating element wrapped or looped around the outside of a section of spa water flow pipe to heat the pipe, which in turn, heats the water flowing through that particular section of pipe. Although this method eliminates the need for bulkheads and electrical lines passing through the water retaining surface, this method provides a very inefficient means of heating water due to the minimal amount of surface area contact between the heating element loops and the flow pipe, resulting in most of the heat being dissipated to the surrounding air or insulation. An example of a device that employs this method of heating spa water is disclosed in U.S. Pat. No. 5,434,388, issued Jul. 18, 1995, invented by Kralik et al., entitled ELECTRICAL HEATER FOR MEDIA, PARTICULARLY FLOW HEATER. The '388 Patent discloses a foil or film-like electrical insulation comprising a plastic film or sheet of high temperature-resistant polymide, provided between the hollow body wall and the heating element. The foil insulation adheres to the wall of the heater by pretensions of a heating element thereby creating an elasticity reserve for thermal expansion. Thus, this device discloses an external insulating/heating device that is wrapped around a heater tube.
An example of a variant of the second type of heating method is disclosed in U.S. Pat. No. 5,172,754, issued Dec. 22, 1992, invented by Graber et al., entitled HEAT EXCHANGER FOR RECOVERY HEAT FROM A SPA OR HOT TUB PUMP MOTOR. The '754 patent is a slight variation in that a small flow tube is looped around the water pump motor to capture the heat produced by the pump motor and transfer the heat to the water flowing through the flow tube. This method is inefficient due to minimal contact area between the water and the heating surface.
Other variants on this theme are disclosed in U.S. Pat. No. 5,415,221, issued May 16, 1995, invented by Zakryk, entitled AUTO SWITCHING SWING POOL/SPA HEATER SYSTEM; U.S. Pat. No. 5,199,116, issued Apr. 6, 1993, invented by Fischer, entitled HIGH-EFFICIENCY PORTABLE SPA; and U.S. Design Patent No. D415,264, issued Oct. 12, 1999, invented by Thweatt, entitled WATER HEATER.
The third method of heating spa water is by providing an elongated heat conductive member constructed of a solid heat conductive material, with water passageways equally spaced about a central axis. An elongated electrical heating element runs along the central axis of the heat conductor member, which radiates heat to the elongated heat conductive member, which in turn radiates heat to the water passageways to heat the water flowing there through. An example of this type of heating method is disclosed in U.S. Pat. No. 5,724,478, issued Mar. 3, 1998, invented by Thweatt, entitled LIQUID HEATER ASSEMBLY. This method of heating spa water is inefficient due to the distance between the heating element and the water passageways, and the amount of solid heat conductive material that must be heated in order for heat to radiate to the water flowing through the water passageways. Furthermore, this method is very expensive to manufacture and requires strict dimensional and bore tolerances to maximize the surface contact area to transfer as much heat as possible from the heating element to the flow pipes. The repair cost for this system can be quite costly as well due to the elaborate piping through a solid aluminum conductive member. A similar device for heating spa water is disclosed in U.S. Pat. No. 6,154,608, issued Nov. 28, 2000, invented by Rochelle, entitled DRY ELEMENT WATER HEATER.
Other relevant devices and methods for heating spa water are disclosed in U.S. Pat. No. 4,529,033, issued Jul. 16, 1985, invented by Blum, entitled HOT TUB HEATING SYSTEM; U.S. Pat. No. 4,150,665, issued Apr. 24, 1979, invented by Wolfson, entitled HEATER FOR HOT TUBS AND STORAGE TANKS; U.S. Pat. No. 4,381,031, issued Apr. 26, 1983, invented by Whitaker et al., entitled SPA-DOMESTIC HOT WATER HEAT EXCHANGER; and U.S. Pat. No. 5,946,927, issued Sep. 7, 1999, invented by Dieckmann et al., entitled HEAT PUMP WATER HEATER AND STORAGE TANK ASSEMBLY.
Accordingly, there is a substantial need in the art for improved spa heater devices that: (1) provide efficient heating of spa water by direct contact of the heating element with the spa water; (2) provide a smooth seamless inner heating surface without the need to pass electrical leads into the wet region of the heater, thereby eliminating the need for bulkhead fittings and reducing the risk of leaks; (3) do not expose the heating elements to high temperature, chemically treated water, thereby eliminating the risk of corrosion; (4) is made by fusing and bonding components together without welds and seams, thereby reducing seam leaks and fatigue stress cracks; (5) are easy and inexpensive to manufacture; (6) can be used with electrical, electromechanical, and mechanical control systems for spas; and (7) can be retrofitted into existing spa applications.